Wireless charging systems have been developed that charge batteries installed in vehicles such as electric vehicles and hybrid vehicles via a wireless connection without the use of plugs. Accurate alignment of power receiving devices on the vehicle side with power transmission devices on the ground side is important to implement efficient charging in such wireless charging systems. Patent Literature 1 describes a system for assisting in aligning a power receiving device on the vehicle side with a power transmission device on the ground side, the system using weak excitation to calculate a distance between the coils and displaying the result thus obtained.